When carrying out manufacturing processes in vacuum environments, it is frequently useful or necessary to employ a small or "miniature" mass spectrometer to indicate the gas species present in the rarified atmosphere within a process zone. A miniature mass spectrometer is able to operate at higher absolute pressures (i.e., not as much vacuum) than a conventionally sized spectrometer, thereby being useful for monitoring some processes, such as sputter deposition of thin films, which cannot be monitored by conventional equipment. Such a mass spectrometer is commonly attached directly to the pressure vessel and operates in the vacuum which is generated by the process system. Mass spectrometers designed for this purpose frequently include a secondary sensing apparatus for indicating the operating vacuum level, such as a total pressure collector or a vacuum gauge, in addition to the primary sensing apparatus for indicating the partial pressure of interest.
Referring to FIGS. 1 and 2, a mass spectrometer 10 of this type includes a dual ion source 16 in which a total pressure (ion) collector 22 and an ion analyzer 18 are oppositely disposed relative to a common ionization volume 26 in which the ions are generated. The ions are generated by heating of respective filaments 24, the ionization volume 26 being operated at a positive potential by biasing an electrode, such as an anode 36, typically in the 80 to 200 volt range with respect to ground, so that positive ions are attracted to the total pressure (ion) collector 22 and the ion analyzer 18. Focus lenses or plates 25, 27 having an opposite negative potential are used to accelerate the ions into movement to the ion analyzer 18 and the total pressure (ion) collector 22, respectively.
The total pressure (ion) collector 22 typically consists of an ion collector electrode 37 having a facing collector surface 21, incorporated with the ion source 16, with suitable electronic circuits to amplify and measure the electric current thus collected based on the collection of generated positive ions from the primary ion beam 34. When calibrated with a reference vacuum gauge, the current collected by the total pressure collector 22 can be used to indicate the degree of vacuum available. Ions strike the facing surface 21 of the collector electrode 37 with sufficient energy to cause the emission of significant quantities of electrons, known as secondary electrons. This well known effect is described in publications, such as Methods of Experimental Physics, vol. 4, Academic Press (1962), the contents of which are herein incorporated by reference.
In brief, the ion analyzer 18 collects and analyzes a first portion of the produced ions to determine a partial pressure for a selected gas species within a sample gas. As described herein, the ion analyzer 18 is a mass filter, such as a quadrupole mass filter, which separates the ions, allowing only those ions having a predetermined mass to charge ratio to pass therethrough to an ion detector 20. The oppositely disposed ion collector 22 collects a second portion of the produced ions from a secondary ion beam 34 to determine a total pressure of the gas sample. The secondary ion beam 34 is not segregated and is representative of the entire gas sample.
The ion detector 20 includes means for collecting the selected ions passing through the ion analyzer 18. The ions are collected and converted to an electric current which can be externally measured by an arranged amplifier and indicator to indicate the quantity of ions collected.
Ion detectors usually contain a combination of a Faraday collector (hereinafter also referred to as FC) and an electron multiplier (hereinafter also referred to as EM) to allow selective operation based on advantages found in each. As is known, a Faraday Collector is a conductive plate or electrode which is attached to ground potential. Positive ions striking the plate are neutralized and draw current from circuitry attached to the electrode. The current flow resulting is exactly equal to the incident ion current. An electron multiplier includes an element which draws the positive ions based on a negative high voltage bias. When an ion strikes a first surface of the EM, one or more secondary electrons are emitted. These electrons are further accelerated to a second and subsequent surfaces, causing the emission of further electrons, the process repeating itself until a stream or pulse of electrons is created which is directed to an electron collector, such as a Faraday Cup. As such, the output from a Faraday detector is positive, while the output of the EM is negative. The advantage is an increased sensitivity, particularly at lower pressures for EMs as opposed to FCs, more advantageously used at higher pressures, for example. Other reasons and advantages are known for each mode of operation to those of sufficient skill in the field. Therefore, no further discussion is required, except as applicable to the present invention.
The ion collecting surface of the total pressure (ion) collector 22 faces the ionization volume 26. It has previously been determined that some of the emitted secondary electrons can be accelerated back into the ionization volume, a portion of which pass though the mass analyzer because the electrons have sufficient velocity to transit the length of the analyzer during a small period of the analyzer selection cycle when the separating voltage is at or near zero.
The effect of the secondary electrons produces a negative baseline effect on the output of the ion detector. As described in U.S. Pat. No. 5,834,770, herein incorporated by reference in its entirety, an ion collector has been designed which deflects a substantial portion of secondary electrons produced by ion bombardment with the ion collector away from the ionization volume.
It has been further determined, however, that due to the amount of energy of the ions (typically on the order of 80 eV or more) accelerated into collision with the ion collecting electrode 37, that photons or other energetic uncharged particles can also be produced. Photons are also emitted from the ionization volume by gas molecules which are excited by the incident electron beam. Some of the photons shine through the ion analyzer 18 to the ion detector 20. Additionally, ions moving through the ion analyzer 18 can be neutralized and retain kinetic energy. The result of photons or other energetic neutral particles impinging onto the conducting Faraday surface is the creation of additional electric current which can not be discriminated from incident ion current. The effect is pressure dependent; that is, more photons are produced with an increasing number of ions contacting the total pressure collector 22, and uniformly affects the baseline in a positive sense, as shown by comparing the graphical outputs illustrated in FIGS. 3(a) and 3(b).
FIG. 3(a) illustrates a spectrum of mass (amu) versus current taken at a 10 milliTorr for nitrogen using the system illustrated in FIGS. 1 and 2. FIG. 3(b) illustrates a similar spectrum taken under the same conditions, but having first removed the total pressure collector 22. The results are fairly pronounced; for example, at mass 21 the baseline current shown in FIG. 3(b) is reduced by a factor of 0.001 when the ion collecting electrode 37 is removed.